For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Game On
Lawyer Atticus Finch is tapped to defend a black man, Tom Robinson, against the rape accusations of a white woman. Instead of giving in to the norm of his small Southern town and phoning in the defense to make sure that Tom goes to jail, Atticus is all, "Imma tell you how it's going to be: I'm going to use all my laywering skills to make sure this man gets a fair trial, black or not black."
Unsurprisingly, the town is not happy about this. The Finch kids, Jem and Scout, endure a lot of playground (and adult) ugliness, but they stand by their dad.
Act II
Trial by Jury
It's not exactly a jury of Tom's peers, since black men aren't allowed to serve on juries. (Neither are women—of any color.) Despite Atticus's impressive lawyering and Tom's clear innocence, the jury finds Tom guilty. But at least they debate for over three hours? Small victories. Or something.
Act III
Things That Go "Boo" in the Night
Mr. Ewell, the dad of the (lying) white woman who accused Tom of rape, isn't too happy with Atticus, so, obviously, he attacks the guy's innocent children one dark night. Mysterious neighbor Boo Radley steps in to save the day and kill Mr. Ewell, leaving Jem and Scout much sadder and wiser kids.